INSIGHTS | PERSPECTIVES
r-process element creation in the universe.
If true, we have some ancient NS merger
in the solar neighborhood to thank for the
world’s store of gold and smartphones.
Merging NSs have long been considered
candidates to produce short-duration extragalactic GRBs (10), whereby high-energy
radiation comes from a narrow collimated
relativistic (velocities very close to c) jet.
However, using observations at radio, x-ray
(11), and g-ray wavebands, the studies reported here rule out such a classical GRB
origin. Instead, they suggest that the merger
remnant was shrouded in a cocoon, a hot ball
of fast-expanding material. Here, the g-rays
(8) arise when the cocoon breaks through
the slower-moving ejecta (see the figure).
NS mergers are a natural consequence of
the discovery of NS binaries that are slowly
spiraling toward each other as the binaries
radiate GW energy (12). But, using known
NS binaries in the Milky Way, extrapolations
of the expected rate of such mergers in the
detectable LIGO/Virgo volume have been uncertain. Ab initio simulations suggest that the
rate depends on how many such systems are
made over galaxy lifetimes, not how many
are produced during recent star formation
(13). If so, many GW/EM events will be detected in massive, old galaxies such as that
which hosted GW/EM170817. Mergers in such
galaxies should typically be offset from their
birth location by thousands of light-years or
more, having been kicked out of their stellar
nurseries during formation.
EM170817 is now tucked away behind
the Sun, and astronomers eagerly await its
return to visibility. Indeed, late-time radio
measurements may spatially resolve the ex-
panded ejecta and help solidify (or refute)
the theoretical models proposed. The dis-
covery of more GW events in the coming
years will help hone our understanding of
the cosmic rate and, inevitably, portray a di-
versity of EM phenomenology. Will face-on
mergers that also produce GWs accompany
a more traditional short GRB and concomi-
tant afterglow? Will highly inclined orbits
lead to any detectable high-energy emis-
sion? We also look forward to other flavors
of events, such as NS-BH mergers (which
may be as abundant). Detecting high-energy
neutrinos from NS mergers could lead to a
deeper understanding of the physics of the
relativistic outflows (14, 15).
With three distinct GW sites operational
at the same time, the GW localization was
remarkably precise, not just on the sky, but
in distance (which is encoded in the GW
waveform). This allowed astronomers to
focus on just a few known galaxies. Not all
events will be so well localized, so close by,
and so bright. Wide-field telescopes like the
Zwicky Transient Facility (16) will be needed
to quickly join the hunt for GW triggers.
For those with visible-light instruments, it
comes as some relief that EM170817 had appreciable optical emission, not entirely suppressed at early times by line blanketing.
Now that we have at least one exemplar as
a template, astronomers can also search for
untriggered cocoon/kilonovae signatures in
past and future imaging surveys.
As Scott Hughes of the Massachusetts
Institute of Technology noted at a high-
energy astrophysics meeting just 5 days af-
ter GW170817, the “end of the beginning” for
direct GW detections has arrived. However,
these conclusive observations of EM signa-
tures following a GW event is a crucial mile-
stone, allowing in-depth study not just of
the dynamical physics of space and time in
the strong-gravity regime, but also the astro-
physics of the progenitors and the connec-
tion to the well-studied transient universe.
These results are enriching and enlight-
ening indeed—double meanings intended! j
REFERENCES AND NOTES
1. M. M. Kasli wal et al. , Science 10.1126/science.aap9455
2. P.A.Evans et al., Science 10.1126/science.aap9580
3. G. Hallinan et al. , Science 10.1126/science.aap9855
4. D. Coulter et al., Science 10.1126/science.aap9811 (2017).
5. LIGO Scientific Collaboration, Virgo Collaboration, Phys.
Rev. Lett. 10.1103/PhysRevLett.119.161101 (2017).
6. L.-X.Li, B.Paczyński,Astrophys.J.507,L59(1998).
7. B. D. Metzger, Living Rev. Relativ. 20, 3 (2017).
8. V. Connaughton et al. , Gamma Ray Coordinates Net work
Circular 21506 (2017).
9. D. Kasen, N. R. Badnell, J. Barnes, Astrophys.J. 774, 25
10. D. Eichler etal. ,Nature340, 126 (1989).
11. E. Troja et al., Nature 10.1038/nature24290 (2017).
12. R.A.Hulse,J.H. Taylor, Astrophys.J. 195,L51(1975).
13. S.E.de Mink, K.Belczynski, Astrophys.J.814,58(2015).
14. S. Rosswog, Int. J. Mod. Phys. D 24, 1530012 (2015).
15. I.Bartos et al., Phys. Rev. Lett. 110,241101(2013).
16. E. Bellm, S. Kulkarni,
Nat.Astron. 1, 0071 (2017).
J.S.B. is a coauthor on (1). He is supported by the Gordon and
Betty Moore Foundation Data-Driven Discovery Initiative.
Published online 16 October 2017;
1 2 34 5 6
Making a neutron